1. Field of the Invention
This invention relates to valves for controlling the discharge of a particulate material from a supply.
2. Background Art
Controlled discharge of particulate material is required in many, diverse industries. Typically, a supply of the material is carried in a hopper having a bottom, load bearing surface with one or more apertures through which the material is delivered to a point of use.
One known structure to variably control the rate of discharge of material from a supply employs three cooperating plates with selectively registrable apertures. Such a structure is shown in British Patent Specification 967,096, issued to Fordath Engineering. A control shifts one plate between the others to effect a desired degree of registration to thereby control the effective area of the discharge opening and the discharge rate.
Several problems are inherent in such a design. First, the three plates have cooperating, flat surfaces, which must be machined to very close tolerances to maintain slidable face-to-face contact. Material may find its way inbetween the plates and thereby cause wear on the elements so as to prevent close engagement between facing plate surfaces. Also, in a high temperature environment, the plates may deform, so that the cooperating, flat surfaces cannot intimately engage and may bind with each other. In either event, the operation of the valve is adversely affected.
Further, a fairly complicated structure is required to accurately guide relative movement of the plates against each other to control the feed rate.
Another problem with prior art sliding plate valves is that it is possible for material to become wedged between the edges of the apertures of the juxtaposed plates so that the moving plate is jammed. A rigid wedged piece may have to be manually moved before resuming normal operation of the valve.
A still further problem with prior art sliding plate valves is that generally poor transition from the no-flow condition occurs once the plate apertures are registered. At startup, flow generally will not begin or will continue for only a short time, even though the plate apertures are in registation, until the valve structure is vibrated to eliminate bridging of the material and to loosen the material supply.